Spiral imaging is a popular fast MRI (Magnetic Resonance Imaging) data acquisition strategy. Flow artifacts are usually not apparent in images reconstructed using spiral techniques due to inherently nulled gradient moments of the spiral trajectories. However, off-resonance effects may lead to blurring artifacts in spiral imaging. Additionally, specific blurring artifacts due to fat spins may be difficult to correct using conventional spiral off-resonance correction methods.
Spiral Dixon techniques using conventional spatially selective RF (radio frequency) pulses may be based on conventional Dixon techniques in rectilinear acquisitions. These spiral Dixon techniques may achieve unambiguous water-fat signal separation with effective blurring artifact correction, even in the presence of B0 inhomogeneity. In Spiral three-point Dixon (S3PD) techniques, a frequency field map can be generated using methods analogous to rectilinear three point Dixon (3PD) techniques since off-resonance blurred signals do not affect the phase difference between the first and third data sets. De-blurred images may then be reconstructed using the frequency field map. However, three point spiral techniques may be computationally intensive. Thus two point techniques may be employed.
Spiral two-point Dixon (S2PD) techniques may be employed to facilitate unambiguous water-fat decomposition in spiral imaging with fewer computations than S3PD techniques. S2PD techniques may also facilitate correcting for off-resonance blurring artifacts using only two data sets. S2PD techniques may acquire the two data sets with different TEs (echo times). However, direct computation of a frequency field map in S2PD techniques may be complicated by the phase relationship between water and fat spins being disrupted by off-resonance blurred signals. Thus, to achieve both water-fat decomposition and signal de-blurring, off-resonance correction at multiple predetermined frequencies may be required in these conventional techniques. For example, in an S2PD technique that employs multi-frequency testing, several predetermined off-resonance frequencies may be tested to facilitate separating water and fat signals and de-blurring the decomposed images. However, the range of tested frequencies must be large enough to span a full range of anticipated B0 variations, and thus S2PD techniques may also be computationally intensive.
Block regional off-resonance correction (BRORC) techniques may also be employed to facilitate correcting for off-resonance blurring artifacts in conventional spiral acquisitions. BRORC has been used with spiral acquisitions using spatial-spectral RF pulses (SPSP pulses). In BRORC, off-resonance correction proceeds block-by-block through a reconstructed image under the assumption that each small sub-image block has a constant B0 off-resonance frequency. This assumption is valid in most cases since magnetic field inhomogeneities are usually smoothly varying across a field-of-view (FOV). BRORC is typically several times more computationally efficient than the conventional frequency-segmented off-resonance correction with no perceptual difference between the images.